1. Field of the Invention
This invention relates to a furnace for drawing optical fiber and, in particular, to an improved graphite element for use therein. Accordingly, it is a general object of this invention to provide new and improved apparatus of such character.
2. Description of the Prior Art
A graphite heating element has been used for heating an optical fiber preform to its softening temperature so that the preform can be drawn to an optical fiber. A graphite heating element should provide conditions suitable for good fiber diameter control. Optical fibers can be drawn to long lengths from preforms; such lengths can be in the order of one kilometer to ten kilometers, and even longer. The fiber diameter should be maintained to a tolerance of .+-.1% to minimize splice losses. The graphite heating element should be maintained at a constant temperature during operation, since temperature variations can cause changes in fiber diameter. To prevent oxidation of the graphite heating element at its operating temperature (which may be in the neighborhood of 2000.degree. C.) tubular input and exit exhaust tubes are inserted into the respective ends of the element to provide for an inert atmosphere (such as argon) therewithin. The glass preform is inserted into the input exhaust tube and positioned within the element chamber where it is heated, and drawn out of the exit exhaust tube to produce an optical fiber. The exhaust tubes are used to direct proper inert gaseous flow to the hot zone within the element.
A disadvantage of a prior art graphite heating element is that the through hole of the element is significantly larger than the glass preform. Such difference in size adversely affects the fiber diameter control, due to gas turbulence (it is believed) that occurs in the annular space between the preform and the element. It is theorized that the turbulence is due to convective gas motion caused by high temperatures, producing a "chimney effect". Due to the "chimney effect", fiber diameter fluctuations tend to enlarge. The mismatch between the element and the preform measurements causes other adverse effects: Additional electrical energy is required to heat the dead space (and, hence, the preform) between the internal wall of the element and the preform. Additional electrical energy increases operating cost. It is believed that such additional electrical energy applied to the graphite heating element, may severely reduce element life.